Automotive tires change or maintain a driving direction of a vehicle, support vehicle and other weights, and absorb and mitigate shocks from the road, and perform a role of braking and towing.
A tire is formed with complex inner structures, and each structure is, although varying depending on the type, commonly formed with a tread part (tread and undertread), a sidewall part, a shoulder part, a bead part, a carcass part, an inner liner, breakers, a bead wire, a chafer, a bead filler (or apex) and a flipper.
A tire formed with the above-mentioned constitution is manufactured as a complex of rubber, a steel cord and a textile cord, and each part is prepared with different compositions for maximizing the role.
A tire bead filler is a rubber layer performing a role of minimizing bead dispersion and protecting beads by mitigating shocks from the outside, and high hardness and modulus properties are required.
A tire bead filler is generally manufactured by mixing a phenol resin to natural rubber, and using a filler such as carbon black.
As the example, Korean Patent No. 10-0888134 discloses a bead filler rubber composition using carbon black and a phenol resin having a melting point of 60° C. to 70° C., a number average molecular weight of 900 to 950 and a weight average molecular weight of 6,000 to 7,000 with material rubber.
In addition, Korean Patent No. 10-1273263 describes a rubber composition for a tire bead filler enhancing hardness and decreasing a modulus difference with adjacent rubber by including carbon black, cashew-modified phenol resin from which m-cresol is removed, and a methylene donor with material rubber.
However, providing sufficient stiffness to the bead part or a shock mitigating ability are lacked with just the phenol resin, and bead wire deformation, and breakaway occur due to high weight and repeated deformation.
In view of the above, studies have been progressed in a direction of changing tire structures for enhancing hardness and modulus properties relating to the stiffness and the shock mitigation, or providing new bead filler compositions.
As one example, a technology of using styrene-butadiene rubber having a different combined styrene content has been proposed. When the combined styrene content is high, stiffness and hardness are high and stability against heat is high, however, compatibility with other rubber is low, and as a result, properties of a finally obtained bead filler decline. In order to compensate this, styrene-butadiene rubber having a low combined styrene content is mixed thereto. This styrene-butadiene rubber has an advantage of having high compatibility while having low thermal resistance and mechanical properties.
In view of the above, Korean Patent No. 10-0846358 prepares a tire bead filler rubber composition including mixed rubber mixing styrene-butadiene rubber having a combined styrene content of 10% to 15% and styrene-butadiene rubber having a combined styrene content of 20% to 25%, natural rubber, carbon black and aramid fiber in order to increase stiffness of the bead filler. The bead filler rubber composition uses only rubber without using a resin such as a phenol resin, and the disclosure proposes that stiffness is enhanced as well as hardness and modulus properties.
However, although using styrene-butadiene rubber having a different combined styrene content is capable of securing complementary effects in the properties theoretically, it has various problems when used in an actual process.
Styrene-butadiene rubber having a low combined styrene content has a very low polymerization rate when prepared through emulsion polymerization, and has low stability against heat applied in a vulcanization process after mixing causing a problem of declining properties of a finally obtained bead filler.